Impact of biogas crop production on greenhouse gas emissions, soil organic matter and food crop production–A case study on farm level

Use of arable land for energy crop production is already a reality in some countries. To meet future sustainability criteria for biofuel systems, it will be crucial to demonstrate systems which do not negatively affect current food production. This is of special relevance regarding future biofuels from energy crops cultivated on arable land and a potential implementation of so called iLUC – indirect land use change – factors.

On the farm that is the model for this study, soil compaction on the medium to heavy clay soils is a problem. Aware of the problem, three years of meadow fescue for seed production has been integrated in the cereal based crop rotation on the most problematic soils. However, the market for grass seeds being limited, the economic possibilities for integrating ley crops in other parts of the crop rotation is limited in a region with little demand for cattle feed. The approach in the present study was to integrate 1-2 years of ley crops in the crop rotation and to use this as feedstock for biogas production. The effects of this on greenhouse gas (GHG) emissions, soil organic matter and food crop production was evaluated.

The climate benefit is high for all the investigated ley scenarios. Using 20-33% of the 650 ha farm for ley production will give a total avoided GHG emission of 1 650 – 2 580 ton CO2-eq/year for the farm. Instead calculating the GHG emission per GJ fuel, the emission reduction amounts to -131% to -143% when the produced upgraded and compressed biogas replaces fossil fuels.
The climate benefit for ley based biogas production is to a large extent the effect of the biogas produced, 15-24 TJ/yr, when replacing fossil fuels. But equally important is the good effect on farm level with increased soil organic matter content. Apart from the role as carbon sink, the increase in soil carbon levels could be important for future soil fertility on this type of compacted clay soils. In the ley scenarios, the soil carbon content increases steadily from 2% today to 3% within 20-30 years to reach a steady state level of 4.4-5.4%. Here, the possibility of using ley for biogas production could open up for a possibility of integrating ley in the crop rotation in cereal intensive areas. Still, land would be taken out of food/feed production. A yield increase of about 20% would be required to counteract the loss of grain and oil seed in the scenarios with one year of ley in the crop rotations.